One aspect of the present invention relates to a stator for an AC induction motor having a plurality of independently switchable windings, and particularly but not exclusively, start and run windings. Another aspect relates to a method of switching the windings of such a motor, particularly but not exclusively when starting the motor.
A problem associated with induction motors is the high inrush current which occurs when the motor is started. The inrush current is caused by the high initial slip between the rotating magnetic fields of the stator and the stationary rotor, leading to a low initial back e.m.f. The electrical supply and other components must therefore be specified for the high inrush current, even though this only occurs during starting. Hence, a lower inrush current allows more efficient use of the electrical system. A number of methods of reducing the inrush current have been proposed, all of which suffer from drawbacks.
The inrush current can be reduced to some extent by increasing the electrical resistance of the rotor cage, but this impairs the running performance of the motor.
Reduced voltage starters supply a low voltage to the windings on starting, which voltage is increased as the motor accelerates. One example is the star-delta system, in which the motor winding configuration is switched from an initial star connection to a delta connection. However, the current surge during the connection changeover can be as high as the initial inrush current. This surge is caused by a phase difference between the rotor and stator fluxes, which occurs in conditions of low load inertia or high friction.
Primary resistance starters apply an initial electrical resistance in series with the windings, which is reduced as the motor accelerates. This type of starter suffers from problems of heat generation and excess weight.
Many of the problems of the electromechanical starters described above can be overcome with an electronic power controller, but these are generally more expensive and less reliable than electromechanical starters, and may be unsuitable for harsh environments or safety-critical applications.
The document U.S. Pat. No. 4,443,749 discloses a multiple split-phase induction motor having a two-pole start winding, a two-pole run winding and a four-pole run winding. To start the motor, current is supplied to both the start winding and one of the run windings. Hence, a high inrush current is likely to occur. The start winding is disconnected once the motor reaches a predetermined speed.
The document U.S. Pat. No. 5,969,497 discloses a starting circuit for an induction motor in which a stationary flux is applied to the rotor through one of the phase windings and current is subsequently applied through another of the phase windings to generate a starting torque. There is no separate start winding; instead, the different phases of the stator winding are switched independently.
The document WO 99/49563 discloses a dual stator winding induction machine having two separate stator windings with different numbers of poles. The aim of this design is to eliminate magnetic coupling between the two windings and to overcome circulating harmonic currents.
The document GB 2,321,560 discloses a stator with radially displaced sets of windings having a mutually different phase arrangement.
The document GB 1,014,365 discloses a stator with start windings displaced radially inwardly and offset by 90xc2x0 from the run windings.
The document EP-A-0 499 989 discloses a method of starting a squirrel cage motor by progressively connecting winding branches to the supply voltage.
According to one aspect of the present invention, there is provided a stator for an AC induction motor, including a set of start windings and a set of run windings independently switchable from the set of start windings, characterized in that the set of start windings is radially offset from, and has a similar phase arrangement to, the set of run windings.
In one specific aspect, the set of start windings has a higher resistance and preferably a greater number of turns than the set of run windings and is displaced radially outwardly from the set of run windings. The start windings can therefore make good thermal contact with an outer casing through the stator laminations and are able to dissipate heat effectively.
In another specific aspect, the set of run windings is displaced radially from the set of start windings in the same apertures within the laminations of the stator such that the flux linkage of the set of start windings is greater than that of the set of run windings. Hence, the impedance of the set of start windings is substantially higher than that of the set of run windings and reduces inrush current, whereas the low impedance of the set of run windings allows high performance to be achieved.
Preferably, the cross-sectional area of the set of start windings is less than that of the set of run windings.
According to another aspect of the invention, there is provided a method of starting an AC induction motor having a first set of windings and a second set of windings independently switchable from the first set, comprising:
a. initially supplying current to the first set of windings without supplying current to the second set of windings, and
b. subsequently supplying current to the second set of windings while continuing to supply current to the first set of windings, characterized by
subsequent to step a, ceasing to supply current to the first set of windings while continuing to supply current to the second set of windings.
Preferably, current is supplied to the second set of windings after the motor has reached a substantially constant speed when driven by the first set of windings.
In this way, a high inrush current can be avoided by initially supplying current only to the first set of windings, while a high switchover current can be avoided by maintaining phase between the first and second set of windings and hence between the windings and the rotor.
Where the first and second set of windings are not identical, circulating currents will be induced when they are both switched on. However, the resultant loss of operating efficiency and excess heat dissipation is insignificant if both sets are switched on for only a short overlap period. This period may be set at a predetermined value which is selected to be as small as possible while ensuring that there will be an overlap period under worst case conditions and tolerance of the components.
According to another aspect of the present invention, there is provided a switch controller for starting an AC induction motor having a first set of windings and a second set of windings independently switchable from the first set, the switch controller comprising means for:
a. initially supplying current to the first set of windings without supplying current to the second set of windings, and
b. subsequently supplying current to the second set of windings while continuing to supply current to the first set of windings;
characterized in that the means is further arranged, subsequent to step a, to cease to supply current to the first set of windings while continuing to supply current to the second set of windings.